1. Field of the Invention
The present invention relates to a screw fastener system, consisting of at least one screw and of at least one plate with at least one through-bore. The screw itself consists of a screw head and of a screw shank, at least the screw head being provided with a thread. The screw can be inserted multiaxially into the through-bore. This means that the screw can be inserted, not perpendicular to the plate, but obliquely with respect to the plate, and can be locked on the plate. A force-fit connection, or a force-fit and form-fit connection, is obtained between the screw and the plate.
2. Definitions
Through-Bore
Through-bore defines bores that extend from the top of a plate to the underside of the plate. It can be cylindrical or conical in shape. Generally, it can be produced using a drill or a milling cutter.
Through-Opening
Through-opening defines openings that are suitable for receiving a screw with screw shank and screw head. In the through-opening, the screw head establishes the force-fit and form-fit connection with the rest of the plate. The through-opening is generally formed initially by two or more through-bores that are arranged relative to one another in such a way that they intersect. To produce a through-opening, a central bore is first made, and further through-bores are formed from this at regular intervals, such that a structure is obtained which, in plan view, looks like a clover or flower or leaf.
Intersection Line
If several through-bores intersect, then intersection lines form in the area of the through-openings and extend through the thickness of the plate. The intersection lines thus extend into the depth of the through-opening. They are preferably configured such that they extend in the direction of the longitudinal extent of the through-bore. The intersection lines are elements that interact with the screw head to bring about a force-fit and form-fit connection through plastic deformation.
Intersection Surface
The intersection lines defined above then become intersection surfaces, if the through-bore and/or the central bore are conical in shape. The intersection surfaces are elements that interact with the screw head to bring about a force-fit and form-fit connection through plastic deformation. For simplicity, only intersection lines will be discussed below. Unless otherwise stated, however, this is also understood to cover the combination of intersection lines and intersection surfaces or only the intersection surfaces.
3. The Prior Art
Screws and plates, in particular bone plates, of the aforementioned type generally form a fixation system and are mainly used to mechanically stabilize bone fractures. This fixation system consists of a plate, which is provided with at least one through-bore, but generally with several through-bores, said through-bores being designed in each case to receive a screw. The screw itself consists of a screw head and of a screw shank and, on the screw shank, has a thread that is able to be screwed into a bone. The screw head itself also has a thread which, in the assembled state of the fixation system, interacts with the plate in the respective through-bore.
Orthopedic fixation devices can be used both outside and also inside the body. They consist of a plate-shaped structure, which extends over a fracture, for example. Securing means are also provided, which are designed, for example, as screws, bolts, nails or pins. For example, so-called bone plates can be secured on a bone by means of bone screws, by the latter being inserted through through-bores and being screwed into the bone. To ensure that these screws do not come loose, particularly under the effect of forces, a great many mechanisms are known from the prior art which avoid loosening of the screw and, consequently, loosening of the plate-shaped structure. Solutions are also known that permit, between the plate and the screw, a force-fit connection in which the screw is not oriented perpendicular to the plate. An oblique position of such a screw is often desired if, for example on account of the bone structure, it is not possible for the screw to be screwed perpendicular to the plate.
Thus, for example, so-called expansion-head screws are known from U.S. Pat. No. 4,484,570 (SYNTHES LTD (US)) Nov. 27, 1984. A head screw that has been screwed into a plate is spread open by an additional screw element in such a way that, by screwing in the additional screw, a wedging effect takes place within the plate, such that the screw is fixed on the plate.
Another embodiment from the prior art, as set out for example in U.S. Pat. No. 5,954,722 (DEPUY ACROMED INC (US)) Sep. 21, 1999, comprises a plate into which a screw can be screwed multiaxially. The multiaxial aspect has, among other things, the advantage that the screwing-in directions are no longer defined by the plate itself. In this way, depending on the nature of the material, it is no longer essential for a screw to be screwed into a bone perpendicular to the longitudinal extent of the plate. For this purpose, the through-bores are additionally provided with a spherical element, which is mounted so as to be at least partially rotatable within the through-bore. This spherical element is arranged captive within the through-bore and interacts with the screw head of the screw that is to be screwed in. The spherical element orients itself depending on the position and thus establishes a force-fit and form-fit connection with the screw head and the plate.
Another multiaxial design of a plate, which interacts with a specially designed screw, is set out in US 20050165400 A (FERNANDEZ ALBERTO A, UY) Jan. 26, 2004. The orthopedic fixation system comprises a plate, which likewise has one or more through-bores. A screw is also provided, which consists of a screw head and of a screw shank. The screw head is specially designed and has a thread that interacts with threads provided at least partially in the through-bores.
To allow a screw to be screwed in obliquely (and thus not perpendicular) to the longitudinal extent of the plate, provision is made for the through-bore to be specially designed. The through-bores have an hourglass-shaped cross section. This means that, seen in the screwing-in direction, the diameter of the through-bore narrows from an initially wide diameter until a defined plane is reached. Starting from this plane, the diameter of the through-bore widens out again.
The through-bore formed in this way has a thread, which interacts with the thread of the screw head. The thread is specially designed and constitutes a so-called cutting thread. This means that, when the screw head is being screwed in, a mechanical cutting process takes place between said thread and the screw head. To strengthen this cutting process and thereby achieve a wedging of the screw head within the through-bore, such that undesired loosening of plate and screw at a later point is no longer possible, cutting elements preferably made of another material are provided within the through-bore. These cutting elements are let into the circumference of the through-bore and bring about a deformation when the screw is screwed into the through-bore. By means of the spherical shape of the screw head, it is possible to choose a large number of angle degrees that deviate from the line perpendicular to the longitudinal extent of the plate.
DE 202004015912U (AESCULAP AG) Dec. 9, 2004 also discloses a fixation system that consists principally of a plate and of a bone screw. The bone screw itself has a shank, which defines a longitudinal axis, and a screw head, which can be brought into engagement with a bone screw seat. A securing element for securing the connection of bone screw and bone plate is additionally provided, the bone screw being able to be brought from a position of engagement, in which the bone screw is held on the bone plate, to a release position.
The through-bores within the bone plate are preferably oval in shape, thereby permitting multiaxial engagement of the bone screw. On their walls, they have threads that can be brought into engagement with the thread of the bone screw or screw head. Securing of the bone screw is achieved by wedging the bone screw to the bone plate. The through-bores are produced by milling an oval through-opening, which has walls formed perpendicular to the longitudinal extent of the bone plate. The walls additionally have thread turns. The production is therefore expensive and complex. The bone screw itself has to be specially designed and have the special securing mechanism available.
WO 2004/084701 A (SWISS ORTHOPEDIC SOLUTIONS SA (CH); YOUNG ROBERT ALLAN (US)) Oct. 7, 2004 discloses a bone plate having a longitudinal extent. The bone plate itself has several through-openings, formed in each case by two through-bores offset relative to each other. The through-bores are arranged in such a way that their midpoint is arranged on the center axis of the respective bone plate and arranged at a defined distance from one another. The two through-bores intersect each other in such a way that, in plan view, a configuration in the shape of an eight is obtained. This means that a narrowing is provided between the two through-bores that form the through-opening. In the lower area, that is to say the area facing toward the bone, both through-bores have thread turns that are different than each other. On the side facing away from the bone, that is to say the top, the area of aperture of the through-bore, and thus also of the through-opening, is much greater than in the lower area.
In particular illustrative embodiments, the thread turns provided in the lower area of the bone plate are arranged at an angle to one another or obliquely.
As in the other prior art, it is necessary to produce the bone plate in different work cycles. In particular, in the lower area of the bone plate, that is to say the area facing toward the bone, it proves difficult to arrange different thread turns that can then interact with the head of the bone screw, which likewise has a thread. A free multiaxial arrangement of the bone screw is therefore not possible, since, when being screwed in, it inevitably interacts with the thread and thus also assumes the predetermined direction.
DE 20321245U (SYNTHES GMBH (CH)) Jun. 14, 2006 also discloses a bone plate with an underside facing toward the bone and with a top, and with several through-bores, which connect the underside to the top and each have a central hole axis, an inner jacket surface and a thread turn. In a particular illustrative embodiment, the through-opening is formed by a central through-bore with, arranged on the circumference of the through-bore, through-bores arranged at an angle distance of in each case 90 degrees from one another.
This permits a multiaxial arrangement of the respective bone screws, the head thereof each being inserted into the respective through-bore. The respective through-bores likewise comprise thread turns, as has already been described above.
For this reason, the production is also very complex, and the multiaxial direction of the respective bone screw cannot be freely chosen, on account of the corresponding specifications of the through-bores.
Moreover, there is no possibility of securing the respective bone screw, such that undesired loosening is possible at any time, as a result of which there is a danger of the function of the respective bone plate being completely lost.